SX-1250 Volume Pot Help

[Old 96er]

While bringing an SX-1250 back from the dead and restoring it, I’ve discovered that someone replaced the original volume potentiometer at some point. The replacement works, but it’s a little wonky and I want to get the original back in there. I see a potentiometer available for an SX-1280, so my question is, will/can it work? Looking at the service manuals for the two models indicates that the only difference is that the 1280 uses a 100k pot and the 1250 uses a 50k pot. The pins look the same. Anyone have any input?

 

[JHS2RT]

I'd like to know also.

 

[Old 96er]

I'd like to know also.

Well, we’re both about to find out since I pulled the trigger and decided to try it. Once it gets here, I’m going to install it and report my results. Last night while recapping my flat amp board I noticed that the replacement pot is an Alps 100k pot with loudness taps. Wires jumper between the pot and the board. It works just fine, especially after a cleaning and application of some fader lube. However, the shaft is bent ever so slightly and causes the knob to wobble a tad. I’ll post back after I get the 1280 pot installed.

 

[montecarlossfan]

It'll work but your volume control will not exactly match the scale on the receiver... it may start to come in at a higher percentage but should make it to 100% as eventually attenuation is 0 ohms once the wiper gets to the end of its travel...Im sure the circuit could be modified so it works correctly as well.

 

[Old 96er]

Update:
I finally got the potentiometer from a SX-1280 installed today into my SX-1250. I found out right away that the pins don’t match up with those on the flat amp board. The pins for the loudness curve are scootched closer to the other pins on the 1280 pot. The other rows of three match up fine. To make it work, I folded the two pins of the loudness taps 90 degrees and soldered a short piece of jumper wire onto each of the pins. Then, I soldered the other end of the jumper wires to the board. Then I placed the potentiometer onto the board and soldered in the pins directly like normal. I put everything back together and tested it out. It works very well. It’s much better than the dinky little pot someone else bodged in. It’s nice to have the tactile feel of the detents back. My volume control doesn’t operate like an ‘80s boombox anymore and once again feels like a piece of hifi gear. Like montecarlossfan mentioned, the scale won’t be accurate since this is a 100k pot instead of the original 50k pot, but I’m not bothered by that in the least.

 

[rcs16]

Check that components for the loudness contour for the 1280 and 1250 are the same if not use the ones used in the 1280 for that pot.

 

[markthefixer]

Check that components for the loudness contour for the 1280 and 1250 are the same if not use the ones used in the 1280 for that pot.

Yup.

 

[Old 96er]

Check that components for the loudness contour for the 1280 and 1250 are the same if not use the ones used in the 1280 for that pot.

Thank you for pointing this out. I finally got around to changing the components in the loudness circuit of my SX-1250 to match those found in the SX-1280. I ended up changing out C13/14, C15/16, R27/28, and R47/48 with the respective component values used in the 1280. I also removed R29/30 (820k-ohm resistors) entirely since those do not exist in the 1280 circuit. Those resistors shunt the volume wiper to ground.

I notice a slight difference in how the loudness function works. The bass doesn’t seem as extreme now since the change and the effectiveness seems to taper off more as I turn up the volume. Purely anecdotal since I have no quantifiable data or a bone stock 1250 to compare to. Let’s just say I’m completely satisfied and will no longer seek out the original volume pot.